1. Field of the Invention
The present invention relates to clad steel materials of low carbon steel and stainless steel having excellent elongation and corrosion resistance and also relates to the production thereof.
2. Description of Prior Arts
Conventionally, sheet steel materials, as typically represented by sheet steel materials used for automobiles and electrical appliances, are generally given a chemical treatment such as a phosphate treatment, after the press forming, and further applied with a paint coating to provide the required corrosion resistance. And for this purpose, low-carbon Al-killed cold reduced steel sheet materials having excellent deep-drawability have long been used.
On the other hand, austenitic series of stainless steel materials are considered to be most excellent with respect to the corrosion resistance, but they are remarkably more costly than plain carbon steel materials. Therefore, clad steel materials of carbon steel and stainless steel have been conventionally proposed and developed as steel sheet materials satisfying both the production cost requirement and the corrosion resistance requirement.
However, in cases of clad steel materials of carbon steel and stainless steel, the high affinity between carbon and chromium causes migration of carbon from the carbon steel layer to the stainless steel layer, resulting in lowered corrosion resistance. For prevention of the carbon migration, it has been proposed to add carbide formers to the carbon steel, as disclosed in Japanese Patent Publication No. Sho 58-15310, Japanese Patent Publication No. Sho 58-19391 and U.S. Pat. No. 3,693,242. These proposed prior arts, however, are still confronted with problems that greater amounts of carbon and nitrogen are present in the carbon steel and greater amounts of titanium and niobium must be added to the carbon steel so that satisfactory workability cannot be obtained and the production cost is high. Other proposals have been made for preventing the carbon migration, including applying a nickel coating on the interface of the carbon steel layer and the stainless steel layer, and insertion of a nickel foil in the interface. These proposed arts suffer from a problem that the production cost is much increased.
Further these proposed arts rely solely upon the conventional hot pressure bonding method in which the component sheets are assembled and welded in a sandwich form and then hot rolled so that the production yield is low and the cost is high.